1. Field of the Invention
The present invention relates to a field effect transistor in which a channel layer is formed of a Group-III nitride compound semiconductor containing nitrogen and at least one of Group-III elements consisting of the group of gallium (Ga), aluminum (Al), boron (B) and indium (In). The invention-also relates to a manufacturing method for the above type of field effect transistor. More particularly, the invention pertains to the above type of field effect transistor having a gate insulating film between a gate electrode and a channel layer. The invention is also concerned with a manufacturing method for this type of field effect transistor.
2. Description of the Related Art
Gallium nitride (GaN), which is a Group-III nitride compound semiconductor, has a large forbidden band gap, such as 3.4 eV. The indirect transition conduction band is positioned at a level higher than the forbidden band by more than 1.5 eV. The saturation velocity of GaN is approximately 2.5.times.10.sup.7 cm/s, which is higher than other types of semiconductors, such as silicon (Si), gallium arsenide (GaAs), and silicon carbide (SiC). Further, the breakdown electric field of GaN is approximately 5.times.10.sup.6 V/cm, which is greater than SiC and much greater than Si and GaAs by more than one order of magnitude. Thus, it has been expected that GaN can be used as a material for high-frequency, high-temperature, and high-power semiconductor devices.
In recent years, prototypes of semiconductor devices using GaN have been made. Among such devices, the transistors configured in a manner similar to the GaAs-type field effect transistors (FETs) have been reported.
FIGS. 16 and 17 illustrate examples of conventional FETs using GaN. The FET shown in FIG. 16 has the following metal semiconductor (MES) structure (M. A. Kahn, A. P L.,62(15), 1786(1993)). An n-type GaN active layer 63 is formed on a sapphire substrate 1 with an intrinsic-GaN buffer layer 2 therebetween. A gate electrode 7, a source electrode 5, and a drain electrode 6 are disposed on the active layer 63. In contrast, the FET illustrated in FIG. 17 has the following high electron mobility transistor (HEMT) structure (M. A. Kahn, A. P L.,65(9), 1121(1994)). An electron transit layer 73b made from impurity-undoped GaN and an electron supply layer 73a made from n-type AlGaN are sequentially laminated on a sapphire substrate 1. A gate electrode 7 is formed on the electron supply layer 73a, and a source electrode 5 and a drain electrode 6 are disposed on the electron transit layer 73b on both sides of the electron supply layer 73a.
In another example of known FETs having the HEMT structure, the thickness of the AlGaN electron supply layer is decreased, thereby making the threshold gate voltage around 0 V (M. A. Kahn, A. P L.,68(4), 22(1996). This type of FET is referred to as "the enhancement-mode FET".
In the foregoing MES or HEMT-structured FETs, the Schottky barrier at the gate electrode between a metal and a semiconductor is comparatively low, such as approximately from 1 to 1.2 eV. Although this Schottky barrier is slightly greater than that of the GaAs-type FETs (0.7 eV), a large forward gate bias voltage cannot be applied. This shortcoming originates from the operation of the MES-structured FET rather than from the constituent material, i.e., GaN.
In contrast, a metal-oxide-semiconductor (MOS) FET, i.e, metal-insulator-semiconductor (MIS) FET, is used as a silicon (Si) FET. In this type of FET, a gate electrode is formed on a Si layer with a silicon oxide (SiO.sub.2) film therebetween, which serves as a highly insulating film, used as a gate insulating film, and an inversion layer formed at the interface between the SiO.sub.2 film and the Si layer is used as a channel, thereby achieving a large input amplitude.
Consequently, if the GaN-type FET uses a chemically stable gate insulating film which has a potential barrier as high as the SiO.sub.2 film, an input amplitude as large as that of the Si FET can be expected. Accordingly, a great level of output can be obtained from the GaN-type FET due to the synergistic effect between the above-described large input amplitude and the high breakdown voltage inherent in the GaN-type FET.